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Peter Anvin" , Paolo Bonzini , , , Tao Zhang , Jim Mattson Content-Type: text/plain; charset="UTF-8" On Thu Oct 30, 2025 at 4:06 PM UTC, Sean Christopherson wrote: > On Thu, Oct 30, 2025, Brendan Jackman wrote: >> > @@ -160,6 +163,8 @@ SYM_FUNC_START(__vmx_vcpu_run) >> > /* Load guest RAX. This kills the @regs pointer! */ >> > mov VCPU_RAX(%_ASM_AX), %_ASM_AX >> > >> > + /* Check EFLAGS.ZF from the VMX_RUN_CLEAR_CPU_BUFFERS bit test above */ >> > + jz .Lskip_clear_cpu_buffers >> >> Hm, it's a bit weird that we have the "alternative" inside >> VM_CLEAR_CPU_BUFFERS, but then we still keep the test+jz >> unconditionally. > > Yeah, I had the same reaction, but couldn't come up with a clean-ish solution > and so ignored it :-) > >> If we really want to super-optimise the no-mitigations-needed case, >> shouldn't we want to avoid the conditional in the asm if it never >> actually leads to a flush? >> >> On the other hand, if we don't mind a couple of extra instructions, >> shouldn't we be fine with just having the whole asm code based solely >> on VMX_RUN_CLEAR_CPU_BUFFERS and leaving the >> X86_FEATURE_CLEAR_CPU_BUF_VM to the C code? >> >> I guess the issue is that in the latter case we'd be back to having >> unnecessary inconsistency with AMD code while in the former case... well >> that would just be really annoying asm code - am I on the right >> wavelength there? So I'm not necessarily asking for changes here, just >> probing in case it prompts any interesting insights on your side. >> >> (Also, maybe this test+jz has a similar cost to the nops that the >> "alternative" would inject anyway...?) > > It's not at all expensive. My bigger objection is that it's hard to follow what's > happening. > > Aha! Idea. IIUC, only the MMIO Stale Data is conditional based on the properties > of the vCPU, so we should track _that_ in a KVM_RUN flag. And then if we add yet > another X86_FEATURE for MMIO Stale Data flushing (instead of a static branch), > this path can use ALTERNATIVE_2. The use of ALTERNATIVE_2 isn't exactly pretty, > but IMO this is much more intuitive. > > diff --git a/arch/x86/kvm/vmx/run_flags.h b/arch/x86/kvm/vmx/run_flags.h > index 004fe1ca89f0..b9651960e069 100644 > --- a/arch/x86/kvm/vmx/run_flags.h > +++ b/arch/x86/kvm/vmx/run_flags.h > @@ -4,10 +4,10 @@ > > #define VMX_RUN_VMRESUME_SHIFT 0 > #define VMX_RUN_SAVE_SPEC_CTRL_SHIFT 1 > -#define VMX_RUN_CLEAR_CPU_BUFFERS_SHIFT 2 > +#define VMX_RUN_CAN_ACCESS_HOST_MMIO_SHIT 2 > > #define VMX_RUN_VMRESUME BIT(VMX_RUN_VMRESUME_SHIFT) > #define VMX_RUN_SAVE_SPEC_CTRL BIT(VMX_RUN_SAVE_SPEC_CTRL_SHIFT) > -#define VMX_RUN_CLEAR_CPU_BUFFERS BIT(VMX_RUN_CLEAR_CPU_BUFFERS_SHIFT) > +#define VMX_RUN_CAN_ACCESS_HOST_MMIO BIT(VMX_RUN_CAN_ACCESS_HOST_MMIO_SHIT) > > #endif /* __KVM_X86_VMX_RUN_FLAGS_H */ > diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S > index ec91f4267eca..50a748b489b4 100644 > --- a/arch/x86/kvm/vmx/vmenter.S > +++ b/arch/x86/kvm/vmx/vmenter.S > @@ -137,8 +137,10 @@ SYM_FUNC_START(__vmx_vcpu_run) > /* Load @regs to RAX. */ > mov (%_ASM_SP), %_ASM_AX > > - /* jz .Lskip_clear_cpu_buffers below relies on this */ > - test $VMX_RUN_CLEAR_CPU_BUFFERS, %ebx > + /* Check if jz .Lskip_clear_cpu_buffers below relies on this */ > + ALTERNATIVE_2 "", > + "", X86_FEATURE_CLEAR_CPU_BUF > + "test $VMX_RUN_CAN_ACCESS_HOST_MMIO, %ebx", X86_FEATURE_CLEAR_CPU_BUFFERS_MMIO Er, I don't understand the ALTERNATIVE_2 here, don't we just need this? ALTERNATIVE "" "test $VMX_RUN_CAN_ACCESS_HOST_MMIO, %ebx", X86_FEATURE_CLEAR_CPU_BUFFERS_MMIO > > /* Check if vmlaunch or vmresume is needed */ > bt $VMX_RUN_VMRESUME_SHIFT, %ebx > @@ -163,8 +165,9 @@ SYM_FUNC_START(__vmx_vcpu_run) > /* Load guest RAX. This kills the @regs pointer! */ > mov VCPU_RAX(%_ASM_AX), %_ASM_AX > > - /* Check EFLAGS.ZF from the VMX_RUN_CLEAR_CPU_BUFFERS bit test above */ > - jz .Lskip_clear_cpu_buffers > + ALTERNATIVE_2 "jmp .Lskip_clear_cpu_buffers", > + "", X86_FEATURE_CLEAR_CPU_BUF > + "jz .Lskip_clear_cpu_buffers", X86_FEATURE_CLEAR_CPU_BUFFERS_MMIO To fit with the rest of Pawan's code this would need s/X86_FEATURE_CLEAR_CPU_BUF/X86_FEATURE_CLEAR_CPU_BUF_VM/, right? In case it reveals that I just don't understand ALTERNATIVE_2 at all, I'm reading this second one as saying: if cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUFFERS_MMIO) "jz .Lskip_clear_cpu_buffers " else if !cpu_feature_enabled(X86_FEATURE_CLEAR_CPU_BUF_VM) "jmp .Lskip_clear_cpu_buffers" I.e. I'm understanding X86_FEATURE_CLEAR_CPU_BUFFERS_MMIO as mutually exclusive with X86_FEATURE_CLEAR_CPU_BUF_VM, it means "you _only_ need to verw MMIO". So basically we moved cpu_buf_vm_clear_mmio_only into a CPU feature to make it accessible from asm? (Also let's use BUF instead of BUFFERS in the name, for consistency)